The present invention relates to a thin-film deposition method for depositing a thin-film on a substrate. A thin-film deposition method has been widely used for manufacturing an electronic device such as an LSI (large-scale integrated circuits) and a display device such as a liquid crystal display. In addition, the thin-film deposition method may be used for manufacturing a solar cell.
The solar cell technology has been conventionally in practical use for an electronic calculator. The technology is expected to use as electric power generating technology as energy problems increase, as seen in New Sunshine Program of MITI (Ministry of International Trade and Industry, Japan).
The Solar cell is divided into two types. One is a silicon solar cell, while the other one is a compound semiconductor solar cell. The silicon solar cell includes a crystallized solar cell such as a single crystalline silicon solar cell and a poly-crystalline silicon solar sell. Further, a great deal of effort has been made to make an amorphous silicon solar cell practical. This is because the amorphous silicon solar cell has an advantage of using a thinner semiconductor layer because of a higher light absorption coefficient, as well as a lower manufacturing cost. In addition, the amorphous solar cell is using an abundant gas as a raw material. Contrarily, the crystal silicon solar cell is using crystal silicon as a raw material that is a limited resource.
In manufacturing the amorphous solar cell, it is necessary to deposit a thin-film on a substrate made of glass, metal or resin. Therefore, a thin-film deposition apparatus is used. In the case of the typical amorphous solar cell, technique of plasma enhanced chemical vapor deposition (CVD) using a mixture of silane gas and hydrogen gas is often adopted. For example, a hydrogenated amorphous silicon film is deposited on a substrate by generating HF (high frequency) discharge in a mixture of silane gas and hydrogen gas to decompose the silane.
In thin-film deposition apparatuses, a temperature of a substrate that is maintained at a specified value during deposition, hereinafter called “deposition temperature”, is often higher than a room temperature. In CVD, the deposition temperature is set higher than the room temperature on purpose that the final reaction could take place by thermal energy, or, the deposition rate and the film quality could be improved. In this case, it is required to provide a process of heating the substrate prior to the deposition.
A heat chamber having a radiation lamp-heater therein is usually used for heating the substrate. The heat chamber is connected air-tightly with a deposition chamber through a valve. The substrate is heated in the heat chamber up to the deposition temperature in vacuum, and is transferred to the deposition chamber for the film deposition. An internal environment of the apparatus is often a vacuum pressure of about 10 Pa or lower. Therefore, the radiation heating is employed, as heat conduction and convection are not expected to be effective in the chamber.
A load-lock chamber is often connected with the deposition chamber so that the deposition chamber is not directly exposed to the atmosphere. A load-lock chamber having the radiation lamp-heater is used as the heating chamber.
However, the above-described radiation heating has problems as follows. First of all, the radiation heating has a high running cost as heating efficiency of the radiation heating is lower than other heating methods. In addition, when a larger substrate is employed, which often happens in the solar cell manufacture, an apparatus cost is increased remarkably because many long radiation lamp-heaters must be provided. Moreover, it is required to consider an issue of energy-payback-time reduction, in which it is necessary to produce a solar cell with energy less than the electric energy the solar cell generates. In this point of view, the radiation heating is not a favorable method because the energy consumption easily increases in the manufacturing process.
In addition, the radiation heating has a problem of having an overshoot when a feed-back-control of the substrate temperature is carried out, because the substrate temperature rapidly rises when irradiation on the substrate starts. That is, the substrate temperature becomes a target value only after exceeding the target value. When the overshoot happens, a thermal stress is generated in the substrate, and the substrate may deform or break, or the stress might remain in the substrate.
In addition, it is important to improve the accuracy of controlling the substrate temperature during the heating to secure the film quality and the reproducibility. However, it is difficult to control the substrate temperature with the high accuracy in the radiation heating. For the high-accuracy control, it is preferable to measure the substrate temperature by a high-performance radiation thermometer. Contrarily, it is difficult to measure the substrate temperature by the radiation thermometer during the radiation heating, because additional radiant ray reflects on the substrate surface other than the infrared ray associated with the substrate temperature.
It is also possible to measure the substrate temperature by a thermocouple. However, in many cases, it is difficult to contact the thermocouple with the substrate. The thermocouple is not suitable for the high-accuracy temperature measurement. Especially, when the substrate is placed in a vacuum, a temperature difference occurs at the contact point between the substrate and the thermocouple because there is no convection to equalize the atmospheric temperature, thereby decreasing the measurement accuracy of the thermocouple.
In addition, the radiation heating has an essential problem in the solar cell manufacturing. In a structure of the solar cell, at least one side of a photovoltaic layer needs an optical transparent electrode. For example, in manufacturing the amorphous silicon solar cell, the amorphous silicon film is often deposited on a TCO (Transparent Conductive Oxide) film formed on the substrate. The TCO film has a characteristic of high infrared-ray reflectivity. Therefore, it is very difficult to effectively heat the substrate having the TCO film using the radiation heating.
Other than the radiation heating, there is a method of heating the substrate using the heat conduction. In this method, a plate with a high thermal conductivity is attached to a backside of the substrate. This plate is called “backing plate”. When the backing plate is heated, the substrate is heated through heat transfer by the conduction from the backing plate to the substrate. However, when the issue of the energy-payback-time reduction is considered, the backing plate is not used, so this method cannot be employed. In addition, it is difficult to contact the baking plate with the substrate sufficiently and uniformly. Therefore, it is difficult to heat the substrate effectively and uniformly.
In addition, in the backing plate method, the substrate is heated only from the backside. As a result, a temperature difference in the thickness direction occurs when a thick substrate is used. The substrate may suffer a thermal deformation before heated up to a required temperature.
There may be another method of heating the substrate from both sides by radiation. Even if this method is adopted, it is difficult to maintain the uniform heating from both sides because the TCO film on one side of the substrate absorbs little infrared ray. Particularly, if the substrate is heated in a vacuum pressure using this method, it is very difficult to heat the substrate uniformly from both sides because there is little heat transfer through the convection and the conduction.
An object of the present invention is to solve problems described above, and to provide a thin-film deposition method having an efficient heating process without using the conventional radiation heating process.
Further objects and advantages of the invention will be apparent from the following description.